Anodizing aluminum foams

ABSTRACT

Strength of metal foams is enhanced by anodizing. The strength increase may not be immediately apparent, but may require aging. Anodization is preferably carried out by applying voltage across a cathode (in contact with the electrolytic bath) and the foam anode and then immersing the anode in the bath. Improved results are also obtained if surfaces not to be anodized are not contacted with the electrolytic bath. The electrolyte can optionally contain a fluorine acid such as HF and HBF4. These tend to make the foam less porous. Preferred foams are made of aluminum base metals.

nited States Patent [191 Niebylski Jan. 22, 1974 ANODIZING ALUMINUM FOAMS [75] Inventor: Leonard M. Niebylski, Birmingham,

Mich.

[73] Assignee: Ethyl Corporation, Richmond, Va.

[22] Filed: Aug. 9, 1971 [21] Appl. No.: 170,335.

[52] 11.8. CI. 204/58 [51] Int. Cl C23b 9/02 [58] Field of Search 204/56, 58; 75/20 F, 20 R [56] References Cited UNITED STATES PATENTS 2,133,996 10/1938 Underwood 204/46 2,040,618 5/1936 Mason et a1. 204/58 3,300,296 l/1967 Hardy et al. 75/20 R 3,297,431 1/1967 Ridgway 75/20 R 2,895,819 7/1959 Fiedler 75/20 R 3,494,840 2/1970 Dedrick et al. 204/58 FOREIGN PATENTS OR APPLICATIONS 482,563 3/1938 Great Britain 204/58 OTHER PUBLICATIONS Anodizing of Al Alloys Hardcoating by Spencer,

Metal Finishing, November 1968, pages 58-59.

Electrodeposition of Cr from CrO Acid Baths by Haring et al., Technologic Papers of the Bur. of Standards, Dept of Commerce, 6-10-1927, p. 436.

Primary ExaminerJohn H. Mack Assistant ExaminerR. L. Andrews Attorney, Agent, or Firm-Donald L. Johnson et al.

[5 7 ABSTRACT Strength of metal foams is enhanced by anodizing. The strength increase may not be immediately apparem, but may require aging. Anodization is preferably carried out by applying voltage across a cathode (in contact with the electrolytic bath) and the foam anode and then immersing the anode in the bath. Improved results are also obtained if surfaces not to be anodized are not contacted with the electrolytic bath. The electrolyte can optionally contain a fluorine acid such as HF and HBF These tend to make the foam less p0- rous. Preferred foams are made of aluminum base metals.

3 Claims, N0 Drawings ANODIZING ALUMINUM FOAMS BACKGROUND OF THEINVENTION metal to foam, and after foaming the resultant mass is' cooled to form a set cellular product. The gas-forming solid may be a metal hydride such as Zrl-I or TiH U.S. Pat. No. 2,983,597.

SUMMARY OF THE INVENTION This invention is directed to foamed metal bodies having an anodized surface. It also pertains to anodizing metal foams.

It has been found that anodizing improves characteristics, such as appearance or strength, of foamed metal. The improvement in strength may not occur immediately upon anodizing. Accordingly, this invention also pertains to a process comprising anodizing a foamed metal and aging the anodized material thereby produced to achieve an increase in strength. The aging can be conducted under ambient atmosphere conditions. In addition to this process, the invention pertains to the foams produced thereby.

It has also been discovered that use of fluorine containing acids makes the anodized surface of a metal foam less porous.Accordingly, this invention also encompasses a process for anodizing a metal foam in the presence of a process improving quantity of a fluorinecontaining acid. Typically the acid is selected from hydrofluoric and fluoboric acid and such substance is used in conjunction with a polybasic acid which has oxygen in the anion portion derivable therefrom. This invention also pertains to the foams produced by the use of such fluorine containing acids in the electrolytic bath.

DESCRIPTION OF PREFERRED EMBODIMENTS TABLE I Metal Preferred Most Preferred Magnesium 2-10 per cent 4-8 per cent Titanium 0.5-2.5 per cent 0.8-1.2 per cent Copper 2.5-35 per cent 8-12 per cent Zinc 3-15 per cent 8-12 per cent Manganese 0.4-1.5 per cent 0.4-0.8 per cent Tin 0.4-2 per cent 1-2 per cent Silicon 0.4-l2 per cent 0.4-2 per cent For the purposes of this invention, substantially pure aluminum refers to aluminum having a purity of at least 98.5 (weight) percent, and aluminum base metal refers to such aluminum and aluminum alloys wherein the aluminum content is at least about 65 (weight) percent.

The following alloys yield foams suitable for this invention when used in a process employing a titanium or zirconium hydride as a blowing agent. Suitable techniques are the processes of the prior art set forth in the patents cited herein in the section Background of the Invention. Moreover, said alloys yield suitable foams when the molten alloy is made more viscous by a suitable viscosity-increasing agent.

Alloy 7075 (1.6 per cent Cu, 2.5 per cent Mg, 0.3

per cent Cr, 5.6 per cent Zn, remainder A1) 2024 (4.5 per cent Cu, 0.6 per cent Mn, 1.5

per cent Mg, remainder Al) 5086 (0.45 per cent Mn, 4.0 per cent Mg, 01

per cent Cr, remainder Al) 6063 (0.4 per cent Si, 0.7 per cent Mg, remainder Al) Almag 35 (6-8 per cent Mg, in A1) 1000 series (99.6 per cent minimum Al) A1 2011 (5.5 per cent Cu, 0.5 per cent Pb, 0.5 per cent Ni, remainder A1) 7001 (2.0 per cent Cu, 30 per cent Mg, 7.5

per cent Zn, 0.3 per cent Cr, remainder A1) 2218 (4.0 per cent Cu, 1.5 per cent Mg, 2 per cent Ni, remainder A1) 3005 (1.2 per cent Mn, 0.4 per cent Mg,

remainder A1) 4042 (12.2 per cent Si, 0.9 per cent Cu, 1.]

per cent Mg, 09 per cent Ni, remainder A1) 4043 (5 per cent Si, 95 per cent A1) 8280 (1.5 percent Si, 1.0 per cent Cu, 0.5 per cent Ni, remainder Al) Magnalium per cent Al, 30 per cent Mg) The techniques of this invention may be applied to foams of other anodizable metals such as magnesium, beryllium, and the like. In a preferred embodiment, the aluminum foam is made from an alloy containing about 6 to about 8 weight percent magnesium, the remainder being essentially aluminum.

Properties of anodic coatings are affected by alloy composition. Magnesium, copper, and zinc dissolve and/or oxidize at the same rate as the aluminum matrix and are preferred alloying elements.

For this invention the density of the metal foam is not critical. For aluminum base foams, it is preferred to use those having a density of from about 8 to about 35 pounds per cubic foot. Likewise, the process of this invention is not dependent upon any pore size. Thus, the process can be conducted on a skin surface produced from a mold as well as the skin surface obtained by the foam casting technique in Bjorksten U.S. Pat. No. 3,305,902. In addition to these surfaces, the process of this invention is applicable to bare metal foams, i.e. to surfaces of the pores of the foams themselves. There is no criticality in the size of the pores. It is preferred to use foams with average pore size of up to one-eighth inch in diameter. Larger and smaller pores can be treated. Thus, foams having pores of from one thirtysecond inch or one sixty-fourth inch or smaller to pores of one-fourth to three-eighths inch or larger can be used. A preferred range within these is from about one thirty-second inch to about one-eighth inch.

Before anodizing it is not critical but usually desirable to treat the surface by precleaning mechanically or chemically. The precleaning practice will vary depending on the condition of the metal surface, the thickness thereof, and its composition. In many instances a degreasing solvent can be used; carbon tetrachloride is a non-limiting example. In many instances it is desirable to follow degreasing with a caustic or acid treatment. A typical caustic treatment is treating with 5 percent NaOI-I at 140F. up to 60 seconds and then rinsing in I cold water. Typical acids suitable for precleaning are I-INO and HF either alone or in mixture, or by using one acid followed by the other. Acid treatment up to 5 minutes is common, as is a following rinse in hot or cold water. Dilute acids are commonly employed. Caustic or acid treatment can be used without prior degreasing if so desired.

In some instances, it is desirable to pretreat the metal body or foam by wire brushing by some mechanical technique. In some cases this can be conducted in lieu of degreasing.

In some instances, burnishing, or other technique which causes plastic flow of a metal at its surface is a desirable treatment prior to anodizing.

As appreciated by skilled practitioners, anodizing is an electrolytic oxidation process. in which a metal surface, when used as an anode, is converted to a coating having desirable protective, decorative, and other properties. With aluminum, the anodic coating is essentially a coating of aluminum oxide.

For the process of this invention, the type of cathode is not critical. Typical cathodes which may be utilized are made of lead orcarbon (graphite). In this process one may use a stainless steel cathode and it is possible to use walls of a stainless steel electrolytic cell as the cathode in the process of this invention. When using a stainless steel cathode it is usually efficacious to continuously filter the electrolytic bath to avoid contamination from iron-containing particles which may flake off the cathode into the bath.

In the anodizing process of this invention, an aqueous sulfuric acid solution is typically employed as the electrolyte. The concentration of sulfuric acid is not really critical. It is only necessary to use a concentration which is sufficiently high to allow a sufficient thickness of anodic coating to be obtained in a reasonable time. Preferred concentrations are not so high as to cause an untoward amount of decomposition of the foam. Thus, for example, one may utilize sulfuric acid concentrations of from about I to 65 weight percent. In many instances, the amount of decomposition in the higher amounts within this range may be quite extensive and accordingly for many foams, about 25 weight percent concentrations are the practical upper limit, especially if a fluorine containing acid is not used in conjunction with the sulfuric acid. For most aluminum-based foams it is preferred to conduct the process using sulfuric acid concentrations which do not exceed about 20 weight percent. A most preferred range is from about 2 to about 7 weight percent.

Although sulfuric acid is the electrolyte of choice the process of this invention does not critically depend upon usage of this substance. Other electrolytes such as chromic acid, oxalic acid, phosphoric acid, and some other materials can be utilized. Thus, the process of this invention is conducted in the presence of polybasic acids with oxygen in the anion portion derivable therefrom. For example, the process of this invention can be conducted employing the 5 to 10 weight percent aqueous solution of chromic acid, I to 5 percent solution of tartaric acid or a 1 to 5 weight percent solution of oxalic acid. Concentrations of these acids that are higher or lower than the aforementioned ranges can also be used.

The anodizing process of this invention is conducted at any convenient temperature which yields satisfactory results, that is, the process temperature is not a critical variable. In general, comparatively cool temperatures are employed. It is not necessary to cool the reaction system below ambient but one can do so if that is desired. Ambient temperatures and slightly elevated temperatures can be used. In general, the process of this invention achieves good results if temperatures within the range of from about 20 to about 60 are used. In a more preferred embodiment, temperatures from ambient to 50C. are employed.

The current density employed is not critical. It is only necessary to use sufficient current to afford a satisfactory amount of coating in a reasonable time. There is no real upper limit on the current density. However, if the current gets too high the coating may be deposited so fast that satisfactory properties are not forthcoming. Also, excessively high current densities will induce heating of the solutions; in addition it is not economically attractive. Preferably, one uses a current density in the range of from about 0.5 to about 4.0 amps per square decimeter, most preferably from about 0.5 to about 2.5. For aluminum foams, current densities within the range of from about 0.8 amp per square decimeter to about 1.5 amp per square decimeter afford efficacious results.

Such current densities are from direct current. The direct current can be impressed upon alternating current if that is desired.

The voltage is not critical and whatever voltage is required to allow the desired current to flow is utilized. For laboratory type experiments, good results are achieved using a direct current power supply which can deliver, say, 50 amps at 220 volts.

Some metal foams which are anodizable according to this invention are susceptible to attack by the electrolytic bath employed in the process. To retard the adverse effect of the bath it is desirable to minimize the exposure of the foam to the bath. In a preferred embodiment of this invention, anodizing is initiated substantially instantaneously upon introduction of the foam into the electrolytic solution. This is achieved by attaching the cathode and the foam anode to the source of direct current in such a way that voltage is impressed and then immersing the foam anode into the electrolyte. When the voltage is high enough to cause current to flow, it will do so practically as soon as the anode is in contact with the electrolyte and in this manner anodizing occurs substantially instantaneously. This diminishes any untoward chemical decomposition prior to starting the electrolytic oxidation process. When this embodiment is employed the voltage which is initially impressed across the cathode and the anode is that voltage which will be necessary to force the current to flow to achieve the desired current density.

In like manner, best results are achieved if the voltage is maintained while the anode workpiece is being removed from the electrolyte bath.

It is not necessary for the electrolyte only to contain the acid in aqueous medium. Other materials can be employed together with the acids mentioned above. For example, adjuvants which have been found useful in this invention are fluorine-containing acids such as hydrofluoric and fluoroboric acid. These improve properties to the coating; such as making the anodized coat less porous.

As mentioned above, foamed metals such as aluminum which are used in the process of this invention are susceptible to chemical attack by the electrolytic bath. Thus, it has been found that anodizing can make a foam more susceptible to water absorption. Water absorption may be undesirable in some uses of metal foams. Where it is, the resistance to water absorption to an extent even beyond the resistance of the original foam can be improved by conducting the anodization in the presence of such fluorine-containing acids.

This effect is illustrated by the data in the table following:

containing acid does not prevent the strength increase discussed below.

Unlike the strength characteristics discussed below, the improvement in water absorption does not require an aging period. The lessening of water absorption is v achieved when anodizing in the presence of other poly- PROPERTIES OF ANODIZED COATINGS ON ALUMINUM FOAMS Anodizing Solution Water Absorption Effects Fluoroboric Acid (48-50 per cent) Wt. per cent per cent Bare Foam Skinned Foam (After 30 min) Base no anodizing 2.33 on" :i

3 0 l5 2 7 L5 3 l Dark Stain Dark Stain Med Dark Stain Light Stain No Stain Light Stain No Stain Contact Angle of H 0 on Surface A B C To develop the data in the above table, foam samples 4 X 2% X it inch were employed. Common cores of skinned foam and bare foam samples were used. The data indicates that the bare foam samplev which was not anodized absorbed 10 ml of water in 2-3 minutes and that the same amount of water caused a dark stain after 30 minutes on the reference sample of skinned foam. Using the anodizing solutions set forth, corresponding samples were anodized for minutes at about -35C. using either a 4000 ml stainless steel beaker (a wall of the beaker served as the cathode), or a glass beaker was used with stainless steel cathode inserted into the solution, and a power supply so as to deliver 5-25 amps at 5-30 volts. As can be seen, anodizing using 15, 7, and 3 weight per cent sulfuric acid caused the bare foam to absorb the same amount of water in decreased times. The lower the amount of H 80, the greater time required until complete absorption. In contrast, adding the amounts of fluoboric acid solution (48-50 percent) to make the bath 2, 1.5, and 1 weight percent relative to that solution resulted in marked reduction of staining of the foam skin, and absorption times longer than even with the untreated foam.

The contact angle is a measure of the hydrophobicity of the surface produced by anodization. As indicated in the table. the greatest angle was achieved when the concentration of fluoroboric acid relative to sulfuric acid was lowest.

The ability of fluoroboric acid to confer lesser tendency for water absorption appears independent of the concentration of polybasic acid utilized in the electrolytic bath. Results such as tabulated above are also achieved using hydrofluoric acid. Positive results with fluoroboric acid suggest that fluosilicic acid can also be employed. It has been found that use of such fluorinea range of concentrations of from about 0.001 to about 50 weight percent of the acid. Greater or lesser concentrations can be used if desired. A preferred range is from about 0.05 to about 3 weight percent. It is not necessary for these acids to be solely employed. Mixtures such that the final concentration is as above can also be used.

As an example, a preferred sequence for conducting the process of this invention is as follows.

EXAMPLE 1. Prepare the electrolyte solution and provide means for maintaining its temperature comparatively cool. These means can be a circulation system which takes a portion of a bath continuously through cooling coils, or for laboratory experiments means as simple as utilizing particles of ice in the electrolytic solution can be employed.

2. Impress a voltage from a direct current source across a cathode and an anode to be anodized.

3. Immerse the anode into the electrolyte and pass the current in the appropriate current density for 15-30 minutes.

4. Remove the anode while the current is on, then stop the current and remove the voltage and then rinse the anodizing work in cold water. This water can be at a temperature within the range of from about 5 to about 30C.; water at greater or lower temperatures can be used. This is followed by a rinse in water which contains a basic substance to neutralize any acid which is on the surface of the anodizing work. In many instances, it is desirable to use a succession of rinses. In one embodiment the first rinse has a concentration of base approximately stoichiometric with the electrolytic bath and subsequent rinses have decreasing amounts of base until one or more rinses in substantially pure water are removed.

5. After the cold water rinse the work is immersed in water at a temperature of 150F. to boiling for 20-30 minutes. This serves to seal the surface. In this step, water with a pH of 5.5-6.5 is employed. Sealant solutions containing 2-15 g/l nickel or cobalt acetate of dichromate have also been found to be useful.

6. Following the immersion in hot water, another cold water dip is used.

7. Finally, the work is air-dried.

The above procedure can be used to anodize aluminum base metal foams as well as foams produced from magnesium alloy such as AZ3lB-F, EZ33A, QE22A, HM21A, ZK60A, I-IK31A and beryllium alloys. For the metals which contain from about 65 weight percent aluminum Almag 35, Alloys 7075, 2024, 5085, and the other alloys mentioned above are typical metals from which foams can be anodized according to this procedure. Foams of these metals having a density of from 8 to 35 pounds per square cubic foot can be used.

Variations of the above procedure can be used. For example, during anodizing the voltage can be decreased or increased causing greater or lesser current to flow. This will cause differences in oxide cell size to vary; it being the general rule that the oxide coatings formed with more current will be larger in size. Similarly harder, less porous coats are prepared when lower electrolyte temperatures, and higher current densities and voltages are produced. In general, the process of this invention is used to lay down coats of 0.0005 to 0.010 inch.

The oxide coating can be pigmented generally in accordance with techniques used with non-foamed aluminum. Both organic and inorganic coloring agents can be used. The organic dyes most often used are acid dyes or sodium salts of acids which have comparatively high solubility in water. The surface of the anodic coating is positively charged and attracts the anions of the dye, enhancing deposition. Typical dye classes which can be employed are anthraquinone, azo, and phthalocyanine substances. Typically, after the aluminum foam is anodized it is removed from the electrolyte bath and rinsed in cold water. In many instances a succession of water rinses is used. While the anodized coating is still wet, it is dipped into the dye solution. A]- ternatively, the anodized foam can be first dipped into a basic solution such as a sodium or ammonium bicarbonate. This dipping is followed by water rinsing. The bicarbonate or other basic solution serves to neutralize any acid remaining on the anodic coating. Such acid tends to prevent deposition of the dye.

Typically, dye solutions contain 2 to 6 grams of dye per liter, and they are maintained at a temperature within the range of l25175F. The pH of the dye solution is as recommended by the dye manufacturer. Typically, the anodic coating is immersed in the dye solution about to minutes. After dyeing, the work is sealed by dipping in hot water and rinsed. In many instances, the dye may be dipped into solutions such as nickel or cobalt salt, which fix the dye and minimize dye leaching during the sealing step.

It is not necessary for the dyes to be acids or to be soluble in water. When using a water-insoluble dye, one can remove the anodic coating, dry it, and then immerse it in an organic solvent solution of the dye.

The inorganic dyes utilized are generally referred to a mineral pigments. Sometimes dyeing with inorganic is conducted using two solutions. For example, the anodic coating may be first dipped into a cobalt acetate solution to absorb cobalt ions. Thereafter this can be dipped in a solution containing sulfide anions whereby black cobalt sulfide is produced. As another example, the material may be first dipped into ferric ammonium oxalate which will form an iron oxide and a gold coloration on the anodized parts. Improvement in appearance can be obtained by dyeing the anodized surface.

The electrolyte bath may be agitated during anodizing. For large scale work, injection of compressed air can be used. In some instances, the compressed air should be filtered and/or scrubbed if it appears to contain rust or oil in an amount which would interfere with the process.

In cases where less than all the foam is to be anodized, best results are obtained if the portion not to be analyzed is kept apart from the electrolytic solution. In some instances this, of course, can be accomplished by not immersing the entire anode into the bath. In those instances where this cannot be done, then shrouding means are preferably used to protect the parts not to be anodized. The shrouding means may consist of gaskets or other close fitting articles to be fitted over the parts not to be anodized. In addition, such means may consist of a metal object to be connected to the positive pole and which is in such contact with the work as to (1) allow current to flow between the work and the cathode at an appreciable rate, and (2) which snugly nests the work so as to only expose the portion to be anodized. In another embodiment, the work need not be snugly nested in an anodic connection; rather the work is juxtaposed against a metal piece with sufficient contact to allow appropriate current and voltage between the work and the cathode, and tight-fitting gaskets or other shroud is used to protect the part of the work which is not to be anodized.

In addition to the above, one can coat or mask the portion not to be analyzed. For example, one can coat with a wax, preferably a material which will withstand the aqueous acid, temperature, and electric current to which it will be exposed during use. In addition to wax, one can coat with a plastic according to the teachings in applications, Ser. Nos. 774,756, filed Nov. 12, 1968, and 155,102, filed June 21, 1971. Both applications are by C. P. Jarema and L. M. Niebylski and have a common assignee with the instant application. In addition, one may mask the portion of the surfaces not to be anodized with a masking agent applied (permanently or temporarily) with an adhesive. Suitable adhesives for this purpose are set forth in application, Ser. No. 866,754, filed Oct. 15, 1969, and Ser. No. 149,020 filed June 1, 1971. Both of these applications were filed by M. E. Kucsma and also have an assignee common with the instant application. All of the aforementioned applications in their entirety are incorporated as if fully set forth.

For further elucidation of this invention, the following typical electrolytic solutions are set forth:

Chromic Acid Solution Chromic Acid 5-10 per cent (6.713.4 oz./ga|.) Chloride below 0.2 g]! as NaCI Sulfate below 0.5 g// as H 504 Sulfuric Acid Solution Sulfuric Acid 2-l5 percent by weight Osalie Acid Solution Oxalic Acid 2 oz. per gal.

Sulfuric Acid fluid oz. per gal.

Phosphoric Acid Solution Phosphoric Acid 2-5 percent by weight M mentioned above, anodizing increases the strength of metal foams. ASTM Test C-365-57 can be utilized to demonstrate the increase of strength produced. Utilizing such tests, it has been found that nonanodized foams in general have a lesser strength than those having anodized coats. The increase in foam strength may not be apparent immediately after anodizing but an aging period may be required. A typical aging period is 30 days but 60 days, 4 months, and 1 year can be employed if desired. The aging is conducted by merely following the anodizing coat to be exposed to ambient temperature, pressure and atsmophere. For example, a typical aluminum foam having a density of about 16.2 pounds per cubic foot has a. compressive strength-according to the aforementioned ASTM test-in the range of from about 840 toabout 940 p.s.i. It has been shown that such a foam upon anodization in percent sulfuric acid and one percent HBF, undergo a decrease in strength, for example, of down to the range of 540 to 740 p.s.i. The same foam which has undergone this apparent degradation upon aging about 60 days will then have a strength of from about 940 to 1040 p.s.i. As a further example, it has been found that an aluminum base foam which has not been anodized and which has a density of 19 pounds per cubic foot will have a normal strength of from about l,040 to l,l60 p.s.i. as illustrative by the ASTM test. Such afoam immediately after anodization in 15 weight percent sulfuric acid and one percent HBF, may have a strength of from 680- 880 p.s.i. After about 60 days, however, the same sample will exhibit a strength within the range of from about 1,320 to 1,400 p.s.i. These increases in strength are typical of those which may be achieved utilizing the anodizing technique of this invention.

I claim:

1. As an article of manufacture, an aluminum based metal foam having an anodized surface, said aluminum based metal having an aluminum content of at least about 65 weight percent, the remainder being selected from the class consisting of copper, magnesium, titanium. manganese, zinc. and silicon. said foam having a density of from about 8 to about 35 pounds per cubic foot, said anodized surface having an oxide coat of from about 0.0005 to 0.10 inch in thickness. said coat being prepared by anodization of said foam in an aque ous sulfuric acid electrolyte wherein the sulfuric acid concentration is no more than about 25 percent by volume using a current density of from about 0.5 to about 4.0 amps per square decimeter, said foam being h t s e z by s av ng... inasasaq. 9zm xa s o strength upon aging said foam after anodization for a period of from about 30 to about 60 days.

2. As an article of manufacture, an aluminum based metal foam having an anodized surface, said aluminum based metal having an aluminum content of at least about 65 weight percent, the remainder being selected from the class consisting of copper, magnesium, titanium, manganese, zinc, and silicon, said foam having a density of from about 8 to about 35 pounds per cubic foot, said anodized surface having an oxide coat of from about 0.0005 to 0.10 inch in thickness, said coat to absorb water.

3. A method for reducing the tendency of a metal foam to absorb water, said method comprising anodizing a foam of an aluminum based metal having an aluminum content of at least about 65 weight percent aluminum, the remainder being selected from the class consisting of copper, magnesium, titanium, manganese, zinc and silicon, said foam having a density of from about 8 to about 35 pounds per cubic foot, said method comprising anodizing said foam using a current density of from about 0.5 to about 4.0 amps per square decimeter in an aqueous electrolyte containing a sulfuric acid at a concentration of no more than about 25 percent by volume and additionally containing from about 1 to about 2 weight percent of fluoroboric acid solution of about 48-50 percent for a period sufficient to deposit an oxide coat of from about 0.0005 to 0.10 inch in thickness. 

2. As an article of manufacture, an aluminum based metal foam having an anodized surface, said aluminum based metal having an aluminum content of at least about 65 weight percent, the remainder being selecteD from the class consisting of copper, magnesium, titanium, manganese, zinc, and silicon, said foam having a density of from about 8 to about 35 pounds per cubic foot, said anodized surface having an oxide coat of from about 0.0005 to 0.10 inch in thickness, said coat being prepared by anodization of said foam in an aqueous sulfuric acid electrolyte wherein the sulfuric acid concentration is no more than about 25 percent by volume, said electrolyte containing from about 1 to about 2 weight percent of 48-50 percent fluoroboric acid using a current density of from about 0.5 to about 4.0 amps per square decimeter, said foam being additionally characterized by having a decreased tendency to absorb water.
 3. A method for reducing the tendency of a metal foam to absorb water, said method comprising anodizing a foam of an aluminum based metal having an aluminum content of at least about 65 weight percent aluminum, the remainder being selected from the class consisting of copper, magnesium, titanium, manganese, zinc and silicon, said foam having a density of from about 8 to about 35 pounds per cubic foot, said method comprising anodizing said foam using a current density of from about 0.5 to about 4.0 amps per square decimeter in an aqueous electrolyte containing a sulfuric acid at a concentration of no more than about 25 percent by volume and additionally containing from about 1 to about 2 weight percent of fluoroboric acid solution of about 48-50 percent for a period sufficient to deposit an oxide coat of from about 0.0005 to 0.10 inch in thickness. 